Sound absorbing and heat insulating material, and method of manufacturing same

ABSTRACT

An object of the present invention is, by utilizing carbon fibers as its constituent material, to provide a thermal-acoustic insulation material having excellent properties in durability, compression resilience, lightness, fireproofness, and non-galvanic corrosiveness. The material of the present invention comprises a wool-like carbon fiber aggregate composed of carbon fibers having an average fiber diameter of 0.5 μm to 5 μm and an average fiber length of 1 mm to 15 mm and the contact points of the fibers are bonded together by a thermosetting resin. The galvanic current of the material is 10 μA or lower in a galvanic cell comprising an electrode composed of the thermal-acoustic insulation material, the other electrode composed of an aluminum plate, and an electrolytic solution composed of 0.45 wt. % sodium chloride aqueous solution.

TECHNICAL FIELD

The present invention relates to a thermal-acoustic insulation materialutilizing carbon fibers, and more particularly to a thermal-acousticinsulation material utilizing extra fine carbon fibers with non-galvaniccorrosiveness.

The thermal-acoustic insulation material of the present invention is notlimited to the material used for insulating both sound and heat, but maybe used exclusively for a purpose of absorbing sound as well asexclusively for a purpose of insulating heat.

BACKGROUND ART

Thermal-acoustic insulation materials have widely been recognized as animportant member to construct comfortable and energy-saving housing orto protect humans and/or equipment from harsh external environment.Although natural fibers and synthetic resins have conventionally beenused for thermal-acoustic insulation materials, these materials have aproblem in safety because they are generally flammable and generatetoxic fumes in case of fire.

For that reason, inorganic materials have recently been used as analternative to natural fibers and synthetic resins. In particular,having favorable properties such as nonflammability, formability, andmountability, glass fibers have been widely used as a raw material forthermal-acoustic insulation materials.

However, since glass fibers have a specific gravity of approximatelyfrom 2.4 to 2.6 g/cm², which is quite large, the thermal-acousticinsulation material made of glass fibers does not exhibit sufficientquality in acoustic and thermal insulation per unit mass. In addition,since glass fibers gradually deteriorate by absorbing moisture and theydo not have sufficient mechanical strength, the thermal-acousticinsulation material made of glass fibers does not retain enoughdurability.

Meanwhile, demand for thermal-acoustic insulation materials having amplesafety and high quality has immensely been increasing. For example, inhigh-speed transport means such as high-speed train cars, aircraft,spacecraft, and the like, high-speed and comfortableness required forsuch means of transport inevitably entail the demand forthermal-acoustic insulation materials having such properties as beinghighly safe, tough and lightweight. Specifically, the requirementsbesides excellent acoustic and thermal insulation quality are 1) to belightweight, 2) to have excellent fire-resistance, 3) not to generatetoxic fumes in case of fire, 4) to have no corrosiveness to structuralmaterials, 5) to have excellent mechanical strength and compressionresilience, 6) to have excellent abrasion resistance, 7) to have littlehygroscopicity, and 8) to have good mountability, and so forth.

As previously mentioned, conventional thermal-acoustic insulationmaterials including the materials made of glass fibers are not reliableenough for the aforementioned uses. Therefore, there has been awaitedthe development of a highly reliable thermal-acoustic insulationmaterial usable for the above purposes.

DISCLOSURE OF THE INVENTION

In view of the above-mentioned problems, an object of the presentinvention is, by utilizing carbon fibers, to provide a thermal-acousticinsulation material which excels not only in acoustic and thermalinsulation quality, but also in durability, mechanical strength,compression resilience, lightness, chemical stability, nonflammability,non-hygroscopicity, and such properties that the material do not emittoxic fumes in case of fire, as well as non-galvanic corrosiveness, andnon-electrical conductivity.

Carbon fibers generally have high electrical conductivity, excellentantistatic property, and relatively large electromotive force. Thesecharacteristics of carbon fibers have been applied to antistaticpurposes and the like, but regarded as undesirable for a material forthermal-acoustic insulation materials.

The reasons are as follows. If a thermal-acoustic insulation material iscomposed of a material having high electrical conductivity, the materialitself is likely to become a cause of short circuits. Also, if thefragments of the material fall off from the material and then float inthe air, the fragments can go into electric circuits and the like, andthus become a cause of short circuits. Moreover, if the material haselectromotive force, there is a possibility that the material causeselectrochemical reactions to other members surrounding the material andconsequently causes galvanic corrosion to the other members.

One of the objects of the present invention is, as mentioned above, toprovide a thermal-acoustic insulation material practically mountable tohigh-speed train cars, aircraft, and the like. These means of transportgenerally have metals as their main constituent materials and have agreat deal of electric wiring. The purposes of the present invention,therefore, cannot be attained by merely employing carbon fibers in placeof glass fibers as a main constituent material in manufacturingthermal-acoustic insulation materials.

Using carbon fibers, the inventors of the present invention have madeintensive studies on the methods that can resolve the aforementionedproblems. As a result, the inventors have found that the strength of athermal-acoustic insulation material and its acoustic and thermalinsulation quality as well as its galvanic corrosiveness are improved byappropriately setting carbonizing temperatures for carbon fibers. Inaddition, the inventors have found that galvanic corrosion caused by athermal-acoustic insulation material comprising carbon fibers as amaterial can practically be prevented when a galvanic current value ofthe thermal-acoustic insulation material is controlled at 10 μA or less.On the basis of this learning, the inventors have completed the presentinvention with a group of the inventions described hereinafter.

It is to be understood that electrical conductivity of a material is notdirectly related to galvanic corrosiveness of the material. The galvaniccurrent value specified herein therefore has great significance since itis an important condition required for a thermal-acoustic insulationmaterial having non-galvanic corrosiveness.

The present invention comprises the following inventions.

(1) Invention 1 is a thermal-acoustic insulation material, comprising awool-like carbon fiber aggregate which is composed of carbon fibershaving an average fiber diameter of 0.5 μm to 5 μm and an average fiberlength of 1 mm to 15 mm, and wherein the fibers are bonded together by athermosetting resin

(2) Invention 2 is a thermal-acoustic insulation material as in theinvention 1, wherein a galvanic current is 10 μA or lower in a galvaniccell which comprises an electrode composed of the thermal-acousticinsulation material, the other electrode composed of an aluminum plate,and the electrolytic solution composed of 0.45 wt. % aqueous sodiumchloride solution.

(3) Invention 3 is a thermal-acoustic insulation material as in theinventions 1 or 2, which has a bulk density of 3 kg/m³ to 10 kg/m³.

(4) Invention 4 is a thermal-acoustic insulation material as in one ofthe inventions 1 to 3, which has a maximum tensile strength of 1.0 g/mm²or higher.

(5) Invention 5 is a thermal-acoustic insulation material as in one ofthe inventions 1 to 4, which has a compression recovery rate of 70% orhigher.

(6) Invention 6 is a thermal-acoustic insulation material as in one ofthe inventions 1 to 5, wherein a minimum tensile strength of theorthogonal direction to the direction of the maximum tensile strength is0.04 times or higher as the maximum tensile strength and, at the sametime, a tensile strength of the orthogonal direction to both thedirection of the maximum tensile strength and the direction of theminimum tensile strength is 0.76 times or higher as the maximum tensilestrength.

(7) Invention 7 is a thermal-acoustic insulation material as in one ofthe inventions 1 to 6, which has a thermal conductivity of 0.039 W/m·°C. or lower.

(8) Invention 8 is a thermal-acoustic insulation material as in one ofthe inventions 1 to 7, wherein a vertical incident acoustic absorptivityat a frequency of 1000 Hz of the material having a thickness of 25 mm is48% or higher.

(9) Invention 9 is a thermal-acoustic insulation material as in one ofthe inventions 1 to 8 wherein the carbon fibers are derived fromanisotropic pitch obtained by polymerizing condensed polycyclichydrocarbon.

(10) Invention 10 is a method of manufacturing a thermal-acousticinsulation material comprising the following steps of:

-   a spinning step of melting anisotropic pitch obtained by    polymerizing condensed 65 polycyclic hydrocarbon, and then    discharging the melted matter out of a spinning nozzle with blowing    a heated gas from around the spinning nozzle in the same direction    to which the melted matter is discharged;-   a carbon fiber producing step of producing non-galvanic-corrosive    carbon fibers by infusibilizing the spun fibers and then carbonizing    the fibers at not lower than 650° C. and lower then 750° C.;-   a spraying and accumulating step of accumulating the    non-galvanic-corrosive carbon fibers into a wool-like material on a    plane with spraying thermosetting resin solution to the fibers; and-   a heat forming step of forming the accumulated matter with applying    heat.

(11) Invention 11 is a method of manufacturing the thermal-acousticinsulation material as in the invention 10, comprising an accumulatingstep in which the non-galvanic-corrosive carbon fibers are accumulatedinto a wool-like material, and a spraying step in which thermosettingresin solution is sprayed to the wool-like material of accumulatedcarbon fibers.

(12) Invention 12 is a method of manufacturing the thermal-acousticinsulation material as in the inventions 10 or 11, wherein the sprayingand accumulating step or the accumulation step comprises a method ofaccumulating fibers by dropping the carbon fibers opened by the air froma height of at least 100 cm or higher.

(13) Invention 13 is a method of manufacturing the thermal-acousticinsulation material as in the inventions 11 or 12 wherein the wool-likecarbon fiber accumulation has a bulk density of 1.3 kg/m³ or lower.

The tensile strength was measured with a constant-traverse-rate-typetensile tester, at a tensile testing speed of 20 mm/min. The length ofthe specimen was 50 mm and the size of the specimen was 50 mm×50 mm×25mm thick (see FIGS. 15 and 16). The method of the tensile test isdetailed in [The conditions in measuring tensile strengths], which willbe described later.

The thermal conductivity was measured at 22° C., according to ASTM C-518(American Society for Testing and Material: Heat flow meter method).

The vertical incident acoustic absorptivity was measured according toJIS (Japanese Industrial Standards) A-1405.

The compression recovery rate was measured as follows: The thickness ofa thermal-acoustic insulation material specimen having a size of 100mm×100 mm×a thickness of 25 mm was measured after 24 times repeating acycle of applying a pressure of 0.7 kg/cm² to the specimen by usingpressure elements with a diameter of 76 mm from the directions of itsthickness for 30 minutes and then releasing the pressure. The values areindicated in a percentage to the initial thickness (25 mm).

The galvanic current mentioned herein is a value of the electric currentmeasured in a galvanic cell expressed by a cell diagram of: a carbonfiber electrode |0.45 wt. % aqueous sodium chloride solution | analuminum electrode. The detail will be described later.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship of carbonizing temperaturesfor carbon fibers to galvanic currents and galvanic corrosion.

FIG. 2 is a graph showing the relationship between carbonizingtemperatures for carbon fibers and tensile strengths of the singlefibers.

FIG. 3 is a graph showing the relationship between carbonizingtemperatures for carbon fibers and tensile strengths of athermal-acoustic insulation material comprising the carbon fibers.

FIG. 4 is a graph showing the relationship between carbonizingtemperatures for carbon fibers (anisotropic pitch and isotropic pitch)and tensile strengths of the single fibers.

FIG. 5 is a graph showing the relationship between carbonizingtemperatures for carbon fibers (anisotropic pitch and isotropic pitch)and tensile strengths of thermal-acoustic insulation materialscomprising the carbon fibers

FIG. 6 is a graph showing the relationship between diameters of carbonfibers and thermal conductivity of thermal-acoustic insulationmaterials.

FIG. 7 is a graph showing the relationship between diameters of carbonfibers and vertical incident acoustic absorptivities of thermal-acousticinsulation materials at a frequency of 1000 Hz.

FIG. 8 is a graph showing sound absorption characteristics ofthermal-acoustic insulation materials having carbon fibers as its mainconstituent.

FIG. 9 is a graph showing the relationship between bulk densities ofthermal-acoustic insulation materials having carbon fibers as its mainconstituent and their thermal insulating qualities (1/λ).

FIG. 10 is a graph showing the relationship between bulk densities ofthermal-acoustic insulation materials having carbon fibers as its mainconstituents and their thermal insulating qualities per unit bulkdensity ((1/λ)/ρ).

FIG. 11 is a histogram showing the distribution of diameters of theprecursor fibers used in Examples 1 to 6 and Comparative Examples 1 and2, which are described later.

FIG. 12 is a schematic diagram of a galvanic cell.

FIG. 13 is an explanatory diagram to illustrate a galvanic corrosiontest.

FIG. 14 is a concept diagram showing the state of contact points of thecarbon fibers that constitute a carbon fiber thermal-acoustic insulationmaterial. (Contact points: ·)

FIG. 15 is a diagram to explain the method of the tensile strength test(lengthwise direction and widthwise direction).

FIG. 16 is a diagram to explain the method of the tensile strength test(thickness direction).

THE BEST MODE FOR CARRYING OUT THE PRESENT INVENTION

The present invention is detailed hereinafter with reference toexperiments and FIGS. 1 through 13 showing the results of theexperiments. The description hereinafter will clarify the effects of thepresent invention.

Unless specified, the carbon fibers used in the experiments hereinaftercomprised anisotropic pitch as a material, and the fibers had an averagefiber diameter of 1.3 μm (fiber diameter: 0.5 μm to 3.5 μm) and anaverage fiber length of 5 mm (fiber length: 1 mm to 15 mm). Thethermal-acoustic insulation materials used in the experiments (only athree dimensional structure of the carbon fibers was used) were producedfrom the aforementioned carbon fibers bonded together by a thermosettingresin, and the resulting materials had a bulk density of 4.8 kg/m³. Thedetails of the producing method, etc. of the thermal-acoustic insulationmaterials will be described later. The thermal-acoustic insulationmaterial made of the carbon fibers whose contact points are bondedtogether by a thermosetting resin is hereinafter referred to as ‘a CFthermal-acoustic insulation material’. The conventional thermal-acousticinsulation material made of glass fibers is hereinafter referred to as‘a thermal-acoustic insulation material made of glass fibers’. (In thefigures, it is shown as ‘GF’).

First, the methods of the experiments will be given below.

The Method of Measuring Galvanic Currents

FIG. 12 shows a device for measuring galvanic currents. In FIG. 12, 1 isa carbon fiber electrode (one side of the electrodes). For measuring agalvanic current of carbon fibers themselves, 100 mg of the carbonfibers was prepared into an aggregate of 1 mm thick, 40 mm wide and 50mm high, and the resulting aggregate was used as the carbon fiberelectrode. For measuring a galvanic current of a CF thermal-acousticinsulation material, the CF thermal-acoustic insulation material madeinto the same size as the aforementioned aggregate was used as a carbonfiber electrode.

In FIG. 12, 2 is an aluminum electrode composed of aluminum alloy 2024(the other side of the electrodes) with the dimensions of 1 mm thick, 40mm wide and about 50 mm high. A glass fiber cloth 3 a has a thickness of0.2 mm, and is disposed between the carbon fiber electrode 1 and thealuminum electrode 2 to restrict the distance between the twoelectrodes. A glass plate 4 supports a side of the carbon fiberelectrode 1 together with a glass fiber cloth 3 b disposed between thecarbon fiber electrode 1 and the glass plate 4, so that the carbonfibers composing the electrode 1 do not come off.

Electrolyte 5 consists of 0.45 wt. % aqueous sodium chloride solution(200 ml). In FIG. 12, 7 is a zero-shunt ammeter (HM-104, manufactured byHokuto Denko Co., Ltd., Japan), and 8 is a beaker made of glass (300ml). An electrode group 9 composed of the carbon fiber electrode 1, thealuminum electrode 2, the glass fiber cloth 3 a, the glass fiber cloth 3b and the glass plate 4 is immersed in the electrolyte 5, and theelectrode 1 and electrode 2 are connected to the zero-shunt ammeter 7with lead wires 6.

By using the device described above, galvanic currents were measuredwith the ammeter 7 after the electrode group 9 was immersed in theelectrolyte 5 for one hour. The values thus measured were employed asgalvanic currents herein.

The Method of Assessing Galvanic Corrosiveness

Whether a specimen has galvanic corrosiveness or not was assessed byusing a test piece shown in FIG. 13. In FIG. 13, 10 is either awool-like carbon fiber aggregate specimen prepared into a dimension of40×40 mm square and 20 mm thick, or a CF thermal-acoustic insulationmaterial prepared into the same dimension as the aforementionedaggregate. In the same figure, 11 is a plate made of aluminum alloy 2024having a dimension of 40×40 mm square and 1 mm thick, where the surfacethereof is polished so as to be a specular surface and then treated witha solution containing 2% CrO₃ and 2% H₃PO₄ A test piece 12 is composedof the specimen 10 sandwiched by the aluminum alloy plates 11.

The test piece 12 was first left in the room where the relative humiditywas 90% and the temperature was 40° C. for 24 hours, next placed in theroom where the conditions were normal (the temperature: 18 to 27° C.,the relative humidity: 40 to 70%), and left for another 24 hours. Then,the test piece 12 was placed back in the room where the relativehumidity was 90% and the temperature was 40° C. This cycle was repeated15 times (30 days). After the cycles were completed (after 30 days), thespecular surface of the aluminum was eye-assayed. When the surfaceshowed the same state as its initial state, it was defined as ‘nogalvanic corrosion observed (−)’. When a little tarnish was recognizedon the surface, it was defined as ‘a little galvanic corrosion observed(±)’. When the surface was obviously corroded, it was defined as‘galvanic corrosion observed (+)’.

For thermal-acoustic insulation materials, tensile strength, thermalconductivity, vertical incident acoustic absorptivity and compressionrecovery rate were measured according to the methods described above.For single fibers, tensile strength was measured in accordance with JISR-7601. The details of the measurement methods are described in thecorresponding sections herein.

Secondly, referring to the figures, the results of the experiments aredetailed below.

The Relationship Between Galvanic Currents and Galvanic Corrosion

FIG. 1 shows the relationship between the carbonizing temperatures forthe carbon fiber precursors, the galvanic currents of the carbon fibersafter a carbonizing treatment and of the CF thermal-acoustic insulationmaterials utilizing the carbon fibers, and the occurrence of galvaniccorrosion (t). The carbon fiber precursors were produced fromanisotropic pitch obtained by polymerizing condensed polycyclichydrocarbon, spun into an average fiber diameter of 1.3 μm and anaverage fiber length of 5 mm and then subjected to an infusibilizingtreatment.

In FIG. 1, λ-λ shows the galvanic currents of the carbon fibers, x-xshows the galvanic currents of the CF thermal-acoustic insulationmaterials. The x-axis (galvanic currents) in the figure is indicated bya logarithmic scale.

As apparent in FIG. 1, as the carbonizing temperatures rose, thegalvanic currents became exponentially larger. As to the occurrence ofgalvanic corrosion, a little corrosion was observed at a galvaniccurrent of 20 μA, while no corrosion was observed at 10 μA or lower.

Judging from the results above, the occurrence of galvanic corrosion canbe practically prevented by controlling a galvanic current of carbonfibers at 20 μA or lower, preferably at 10 μA or lower. Thecorrespondence between the x-axis and the occurrence of galvaniccorrosion indicates that the carbonizing temperature should becontrolled at 800° C. or lower, or preferably at 750° C. or lower, toproduce the carbon fibers which do not cause galvanic corrosion.

In order to sufficiently carbonize the carbon fiber precursors, however,the carbonizing temperature needs to be 550° C. or higher. If thetemperature is lower than 550° C., it is possible that the carbonizingtreatment is insufficient. For these reasons, the carbonizingtemperature should be controlled within the range of 550° C. to 800° C.,or preferably 550° C. to 750° C.

In the experiment concerning FIG. 1, a little galvanic corrosion wasobserved when the galvanic current of the carbon fibers was 20 μA.However, the above-described test results regarding galvaniccorrosiveness were obtained under such severe conditions that thespecimen was exposed to a temperature of 40° C. and a relative humidityof 90% repeatedly. Compared to this, the conditions under whichthermal-acoustic insulation materials are normally used are not sosevere. It is therefore believed that when the galvanic current iscontrolled at lower than 20 μA, the occurrence of galvanic corrosion canbe prevented.

There was a slight difference between the galvanic currents of carbonfibers themselves (λ-λ) and the galvanic currents of the CFthermal-acoustic insulation materials utilizing the carbon fibers (x-x).This difference is considered to have been the effect of thethermosetting resin contained in the CF thermal-acoustic insulationmaterials. That is, the thermosetting resin worked to lower the galvaniccurrents. For this reason, it is considered that a CF thermal-acousticinsulation material which practically causes no galvanic corrosion willbe obtained if the material is produced from the carbon fibers having agalvanic current of 20 μA or lower. Nevertheless, as mentioned above, itis more preferable to control the galvanic current at 10 μA or lower.

The Relationship of Carbonizing Temperatures to Tensile Strengths andElongations

FIG. 2 shows the relationship of the carbonizing temperatures to thetensile strengths kg/mm²) and elongations of the single fibers. Theupper graph of FIG. 3 shows the relationship between the tensilestrengths in the lengthwise direction of the CF thermal-acousticinsulation materials (g/mm²) and the carbonizing temperatures for thecarbon fibers which constitute the materials. The lower graph of FIG. 3shows the relationship between the compression recovery rates (%) of theCF thermal-acoustic insulation materials and the carbonizingtemperatures.

Concerning the single fibers, the tensile strength values herein weremeasured according to JIS R-7061. However, since it is difficult tomeasure the tensile strengths of extra fine fibers with diameters of 0.5μm to 3.5 μm (an average fiber diameter of 1.3 μm), the results shown inFIG. 2 were obtained according to the following method. First, carbonfibers with diameters of 10 μm to 13 μm were prepared under the sameconditions except their diameters, and the tensile strengths of theprepared fibers were measured. Then, the measured results were convertedinto the values per unit cross-sectional area, and the converted valuesare shown in the figures. Concerning the CF thermal-acoustic insulationmaterials, the materials employed in this experiment were composed onlyof the three-dimensional structures (bulk density: 4.8 kg/m³) of carbonfibers, and the tensile strength values were measured according to theconditions described later in “The conditions in measuring tensilestrength”.

As apparent in FIG. 2, as the carbonizing temperatures became higher,the tensile strengths of the carbon fibers themselves became larger,forming a graph like a linear function. Concerning the elongations, thelargest value was observed at about 650° C., the large elongation valueswere obtained from 625° C. to 800° C., and the values kept unchangedover 800° C.

As apparent in FIG. 3, concerning the tensile strengths of the CFthermal-acoustic insulation materials, the largest value was at around700° C. and the smallest was at around 800° C. (the upper graph in FIG.3).

The relationship between the compression recovery rates and thecarbonizing temperatures for the CF thermal-acoustic insulation materialshowed similar results to the above-mentioned relationship between thetensile strengths and the carbonizing temperatures (the lower graph inFIG. 3).

From these results, it is understood that carbon fibers having a largeelongation value can be produced by the carbonizing treatment at about625° C. to 800° C. It is also understood that if the carbon fibers thusproduced are employed in producing a CF thermal-acoustic insulationmaterial, it is possible to produce the CF thermal-acoustic insulationmaterial having a tensile strength of 1.0 g/mm² or higher, and that theresulting material thereby has an excellent compression recovery rate.

In FIGS. 2 and 3, the relationship between the tensile strengths and thecarbonizing temperatures, and the relationship between the compressionrecovery rates and the carbonizing temperatures both resulted in quadriccurves having maximum values and minimum values unexpectedly, while therelationship between the tensile strengths of the carbon fibers (singlefibers) and the carbonizing temperatures resulted in a graph like alinear function. The reason for this is considered to be that theelongations of carbon fibers significantly affects the tensile strengthsand the compression recovery rates of the CF thermal-acoustic insulationmaterials. Now, the consideration to this matter is given below.

FIG. 14 is a concept diagram showing the contact state of the carbonfibers which constitute a CF thermal-acoustic insulation material(contact point: ·). With reference to FIG. 14, when a CFthermal-acoustic insulation material is stretched toward the directionsindicated by the allows, the shape of the meshes changes and each linesegment which constitutes the meshes turns to the stretching directions.However, because the length of each line segment varies, the linesegments which constitute certain sides of the meshes receive greatertension than those of the others. As a result of this, the line segmentsare cut off, or the contact points (·) bonding the segments together aredetached.

In the case where the meshes are composed of the line segments (i.e.carbon fibers) having large extensibility, even if specific sides (linesegments) are stretched with larger tension than the rest of the sides,the extension in the specific side enables the meshes to endure thetension due to the resultant force % with the others sides. In otherwords, as a result of the meshes resisting the tension by working as anetwork, the cutoffs of the line segments and the detachments of theconnected points ( ) are reduced, and therefore the larger tensilestrength of the whole material is obtained. It is considered that thetensile strength as a network becomes the largest when the elongationand the tensile strength of single fibers are best balanced.

The results shown in FIGS. 2 and 3 indicate that when the carbonizingtreatment is carried out at 700° C., the tensile strength and theelongation of the single fibers are best balanced and thus the tensilestrength of the whole material (the tensile strength of the CFthermal-acoustic insulation material) becomes large, and so does thecompression recovery rate of the CF thermal-acoustic insulationmaterial.

The Difference Between Isotropic Pitch and Anisotropic Pitch

FIG. 4 shows the relationship between the carbonizing temperatures forcarbon fibers and the tensile strengths of the fibers. The fibers usedin this experiment were the carbon fibers manufactured from anisotropicpitch a obtained by polymerizing condensed polycyclic hydrocarbon andthe ones manufactured from isotropic pitch made from coal tar. FIG. 5shows the relationship between the carbonizing temperatures for thosecarbon fibers and the tensile strengths (in the lengthwise direction) ofthe CF thermal-acoustic insulation materials (bulk density 4.8 kg/m³).The CF thermal-acoustic insulation materials used in this experiment areproduced from the above-mentioned carbon fibers.

As apparent in FIG. 4, the carbon fibers manufactured from anisotropicpitch had far larger tensile strengths than the carbon fibersmanufactured from isotropic pitch. Also, as apparent in FIG. 5, therewas no maximum value and minimum value in the tensile strengths of theCF thermal-acoustic insulation materials comprising the carbon fibersmanufactured from isotropic pitch, as opposed to the graph of the CFthermal-acoustic insulation materials comprising the carbon fibersmanufactured from anisotropic pitch. From the results of thisexperiment, it is concluded that the existence of the maximum andminimum tensile strength value is a property particular to the carbonfibers manufactured from anisotropic pitch obtained by polymerizingcondensed polycyclic hydrocarbon. The existence of the maximum andminimum value has therefore a great significance in improving thequality and production efficiency of the CF thermal-acoustic insulationmaterial of the present invention.

The following are concluded from the results of the experiments shown inFIGS. 1 through 5. Anisotropic pitch obtained by polymerizing condensedpolycyclic hydrocarbon should be employed as a material of the carbonfiber precursors in view of the elongation (toughness) of the carbonfibers and the tensile strength of the CF thermal-acoustic insulationmaterial produced from 0.5 the precursors. The carbonizing temperaturefor carbon fiber precursors should be 550° C. or higher but lower than800° C., more preferably 550° C. to 750° C., or most preferably 650° C.to 750° C. If a CF thermal-acoustic insulation material is produced fromthe carbon fibers subjected to the carbonizing treatment at 650° C. to750° C., the resulting material will have a tensile strength of 1.0g/mm³ or higher and a galvanic current of 10 μA or lower. The CFthermal-acoustic insulation material thus obtained practically causes nogalvanic corrosion.

The Relationship of Fiber Diameters to Thermal Conductivity and VerticalIncident Acoustic Absorptivities

FIG. 6 shows the relationship between the thermal conductivity λ (W/m·°C.) of the CF thermal-acoustic insulation material (bulk density: 4.8kg/m³, thickness: 25 mm) and the average diameters of the carbon fiberscomposing the material. FIG. 7 shows the relationship between theaverage diameters of the carbon fibers composing the material and thevertical incident acoustic absorptivities at 1000 Hz of the material.The thermal conductivity and the vertical incident acoustic absorptivityof a conventional thermal-acoustic insulation material made of glassfibers (GF, bulk density: 4.8 kg/m³, average fiber diameter: 1 μm,average fiber length: 1 mm) are also shown in the FIG. 6 and FIG. 7 fora comparison (plot: x).

The values in FIG. 6 and FIG. 7 were obtained according to atrial-and-error method. The reason is that, since the diameters ofmanufactured carbon fibers are distributed as shown in FIG. 11, a greatdeal of ingenuity in manufacture process and trouble in measurementprocess is required in preparing a CF thermal-acoustic insulationmaterial having a specific average fiber diameter.

FIG. 6 shows that the larger the diameter of the carbon fibers is, thegreater the thermal conductivity. FIG. 6 also shows that the CFthermal-acoustic insulation material comprising the carbon fibers havingan average diameter of 5 μm or smaller can attain the degree of thermalinsulation quality equal to or higher than that of the thermal-acousticinsulation material made of glass fibers having an average diameter of 1μm (thermal conductivity: 0.039 W/m·° C.). In other words, whether ornot the average diameter stays within the value limitation of 5 μmdetermines whether or not the degree of thermal insulation quality of aCF thermal-acoustic insulation material is equal to or higher than thatof the material made of glass fibers having an average diameter of 1 μm.In FIG. 6, even when the carbon fibers having larger average diametersthan the average diameter of the glass fibers were employed, the degreeof thermal insulation quality was still higher than that of the materialcomprising the glass fibers. This is surprisingly desirable, sincecarbon fibers having larger diameters are easier to manufacture.

FIG. 7 shows that the larger the average diameter of carbon fibers is,the less the vertical incident acoustic absorptivity. FIG. 7 also showsthat the CF thermal-acoustic insulation material comprising the carbonfibers having an average diameter of approximately 2 μm or smaller canattain the acoustic absorptivity equal to or higher than that of theconventional material made of glass fibers (48% or higher when thethickness is 25 mm). In other words, whether or not the average diameterstays within the value limitation of 2 μm determines whether or not theacoustic absorption quality of a CF thermal-acoustic insulation materialis equal to or higher than that of the thermal-acoustic insulationmaterial made of glass fibers having an average diameter of 1 μm. InFIG. 7, even when the carbon fibers having larger average diameters thanthe average diameter of the glass fibers were employed, the acousticabsorptivities were higher than that of the material made of the glassfibers. Again, this is surprisingly desirable, since carbon fibershaving larger diameters are easier to manufacture.

The above results demonstrate that it is preferable to set an averagediameter of carbon fibers at 5 μm or smaller, or more preferably 2 μm orsmaller. However, in the present state of art, it is extremely difficultto manufacture carbon fibers with an average fiber diameter of less than0.5 μm. A preferable average diameter of carbon fibers is, therefore,0.5 μm to 5 μm, or more preferably 0.5 μm to 2 μm.

Concerning the length of carbon fibers (fiber length), it is difficultto manufacture the carbon fibers having an average fiber length of morethan 15 mm when the fibers are extra fine carbon fibers with an averagefiber diameter of 0.5 μm to 5 μm. Moreover, the fibers longer than 15 mmare not preferable because the fibers tend to be orientedtwo-dimensionally in producing the carbon fiber aggregate. Likewise, ifthe average fiber length is shorter than 1 mm, it is impossible to formdesirable three-dimensional structures because the intertwinement of thefibers cannot develop easily. Further, the fibers with an average fiberdiameter of 1 mm are likely to produce such problems that the fibers areeasily detached from the structures, causing faults in electricappliances surrounding the structures by the fiber fragments enteringthe electric circuits. On the other hand, the fibers having an averagelength of 3 mm to 8 mm are easy to be manufactured and to be orientedthree-dimensionally.

For the above reasons, it is preferable to set an average fiber lengthsat 1 mm to 15 mm, or more preferably at 3 mm to 8 mm.

In FIG. 8, the graphs shows the vertical incident acousticabsorptivities against frequencies concerning the CF thermal-acousticinsulation material comprising the carbon fibers with an averagediameter of 1.3 μm and the material comprising the carbon fibers with anaverage diameter of 13 μm.

From the comparison of the above materials, it is understood that the CFthermal-acoustic insulation material comprising extra fine carbon fiberswith an average diameter of 1.3 μm has excellent acoustic absorptionquality particularly in a high frequency range.

Bulk Density and Thermal Insulation Property

A variety of CF thermal-acoustic insulation materials only different intheir bulk densities were prepared to measure their thermal conductivityλ(λ=W/m·° C., W means ‘watt’). The results of the measurements are shownin FIG. 9 as the relationship between the bulk densities and 1/λ(thermal insulation property). With reference to FIG. 9, therelationship between the bulk density and the thermal insulationproperty (1/λ) of the CF thermal-acoustic insulation material isexplained below. FIG. 9 shows both the results of the CFthermal-acoustic insulation materials (thickness: 25 mm, represented byλ-λ) and of the conventional thermal-acoustic insulation materials whichhas an average fiber diameter of 1.0 μm or 2.5 μm and an average fiberlength of 5 to 15 mm (represented by x).

FIG. 9 illustrates that the larger the bulk density is, the better thethermal insulation property. It is noted that, however, the degree ofthe improvement decreases, as the bulk density becomes larger. It isalso noted that the thermal insulation property of a CF thermal-acousticinsulation material is far superior to that of the material made ofglass fibers when compared at the same bulk density. This indicates thateven with a smaller weight, a CF thermal-acoustic insulation materialcan achieve the same degree of thermal insulation property as that ofthe thermal-acoustic insulation material made of glass fibers. Further,it is noted that, if the CF thermal-acoustic insulation material has abulk density of 3 kg/m³ or higher, the effect of thermal insulation isequal to or higher than that of the material comprising glass fiber withan average fiber diameter of 1.0 μm.

FIG. 10 is a graph in which the x-axis shows the bulk density and they-axis the values obtained by dividing thermal insulation property (1/λ)by bulk density ρ((1/λ)/ρ). This graph illustrates the thermalinsulation property of a thermal-acoustic insulation material per unitbulk density (thermal insulation quality per weight). In FIG. 10, as thebulk density increases, the thermal insulation property per unit bulkdensity decreases forming almost a linear line. This indicates that thesmaller the bulk density is, the better the thermal insulation qualityper weight, and that a CF thermal-acoustic insulation material hasbetter thermal insulation quality per weight ((1/λ)ρ) than the materialmade of glass fibers (x).

Further, the thermal insulation quality per weight of the CFthermal-acoustic insulation material with a bulk density of 10 kg/m³ isalmost equal to that of the material made of glass fibers with a bulkdensity of 6.3 kg/m³ (x). This result demonstrates that if the bulkdensity of a CF thermal-acoustic insulation material is 10 kg/m³ orless, the thermal insulation quality can be guaranteed to be equal to orhigher than that of the material made of glass fibers (bulk density: 6.7kg/m³), which is a typical conventionally-used thermal-acousticinsulation material.

From the results mentioned above, it is concluded that a bulk density ofa CF thermal-acoustic insulation material should preferably be 3 kg/m³to 10 kg/m³.

Compression Recovery

Compression recovery rate is one of the properties that reflectsmechanical strength of a CF thermal-acoustic insulation material. If athermal-acoustic insulation material with a small compression recoveryrate is used under such conditions that the material is affected byvibration and compression force resulting from the vibration, theinitial acoustic and thermal insulation quality will fade away in ashort period. One of the reasons is that if the compression recoveryrate is small, the bulk of the material gradually decreases due to thevibration and compression, which brings about the shrinkage of the innergap of the material and thereby leads to the deterioration of thematerial's quality. Another reason is that the decrease of the bulkcauses a gap in the space in which the material was fitted.

One of the objects of the present invention is to provide a CFthermal-acoustic insulation material having the quality equal to orhigher than that of the conventional material made of glass fibers. Thematerial of the present invention, therefore, has to assure the samedegree of the compression recovery rate as that of the material made ofglass fibers. The typical conventionally-used material made of glassfibers (bulk density: 6.7 kg/³) has a compression recovery rate of 70%.(See Comparative Example 4 in table 4, which will be described later.)

On that account, a compression recovery rate of a CF thermal-acousticinsulation material should be at least 70% or higher, or more preferably85% or higher. The reason is that if the compression recovery rate of aCF thermal-acoustic insulation material is 85% or higher, the materialcan endure the external forces in mounting and manufacturing, and thematerial can also be usable under such conditions that the material isconstantly affected by vibration and compression force.

The Tensile Strength Ratios in Three-Dimensional Directions

A further limiting factor concerning a CF thermal-acoustic insulationmaterial of the present invention will be described below. One of theobjects of the present invention is to provide a lightweight andhigh-quality CF thermal-acoustic insulation material utilizing extrafine carbon fibers. Another object of the present invention is toprovide the material usable under such conditions that the material isconstantly affected by vibration and compression force, for example onhigh-speed train cars and aircraft. However, when extra fine carbonfibers are employed in producing a CF thermal-acoustic insulationmaterial with a small bulk density, the mechanical strength thereofresults in a smaller value than that of the material made of medium finefibers with a large bulk density, and therefore the material made ofextra fine fibers is likely to be inferior in its handleability,mountability, and durability.

In particular, if the strengths in the three-dimensional directions areextremely different in each direction, the resulting material easilybreaks from the weakest direction. Nevertheless, it is difficult torender the absolute strength in each direction very large, as long asextra fine fibers are employed and a small bulk density is required. Thedifference between the tensile strengths in each direction of x-axis,y-axis and z-axis in three-dimensional coordinates (hereinafter referredto as ‘three axes’) should therefore be rendered small so that the CFthermal-acoustic insulation material can retain a small weight and bulkdensity as well as excellent handleability, mountability, durability andcompression recovery rate.

In view of the above reasons, for a preferable embodiment of the presentinvention, the minimum tensile strength in the orthogonal direction tothe maximum tensile strength direction of a thermal-acoustic insulationmaterial should be equal to or greater than 4% of the maximum tensilestrength. The tensile strength in the direction orthogonal to both thedirections of the maximum tensile strength and the minimum tensilestrength should be equal to or greater than 35% of the maximum tensilestrength.

The grounds of these values will be described below with reference totable 1 and table 2. Table 1 shows the measurement results of thetensile strengths of the thermal-acoustic insulation materials onlyvaried in their bulk densities. The values were measured in thedirections of three axes, i.e. their lengthwise, widthwise and thicknessdirections.

Table 2 shows the percentages of the tensile strengths in the minimumtensile strength direction to the tensile strengths in the maxiumtensile strength direction and to the tensile strengths in the mediumtensile strength direction. It also shows the percentages of the tensilestrengths in the minimum tensile strength direction to the tensilestrengths in the medium tensile strength direction.

Normally, the minimum tensile strength direction is the material'sthickness direction, and the maximum tensile strength direction is thelengthwise or widthwise direction. The medium tensile strength directionis the direction that exhibits a tensile strength between the maximumtensile strength and the minimum tensile strength, and it is normallythe widthwise direction.

[The Conditions in Measuring Tensile Strength]

The tensile strengths herein were measured with aconstant-traverse-rate-type tensile tester, in accordance with thefollowing conditions.

(1) The tensile strengths in the lengthwise and widthwise directions

Tensile strength test speed 20 mm/min. Specimen's length 50 mm Specimensize 50 mm × 50 mm and a thickness of 25 mm (see FIG. 15)

-   -   (2) The tensile strengths in the thickness direction

Tensile strength test speed 20 mm/min. Specimen's thickness 25 mmSpecimen size 60 mm × 60 mm and a thickness of 25 mm (see FIG. 16)

The tensile strengths in the thickness direction were measured bypulling the plates attached onto both sides of a CF thermal-acousticinsulation material (the margins in FIG. 16) toward the directionsindicated by the arrows. The thermal-acoustic insulation materials madeof glass fibers, employed as a comparative subject herein, were producedfrom glass fibers having an average fiber diameter of 1 μm and anaverage fiber length of 10 mm.

TABLE 1 Type of insulation Glass fiber CF Bulk density (kg/m³) 5 10 3 57 10 Tensile Maximum 1.0 0.25 0.21 1.5 2.7 5.2 strength in tensile threeaxis strength directions direction (lengthwise) Medium 0.35 0.19 0.211.3 2.6 4.9 tensile strength direction (widthwise) Minimum 0.010 0.0100.012 0.085 0.14 0.15 tensile strength direction (thickness wise)Tensile strength: g/mm²

TABLE 2 Type of insulation Glass fiber CF Bulk density (kg/m³) 5 10 3 57 10 Tensile Minimum to 1.0 4 5.7 5.7 5.2 2.9 strength maximum ratio*Medium to 35 76 100 86 96 94 maximum Minimum to 2.9 5.3 5.7 6.5 5.4 3.1medium *Percentage

As apparent in table 1, when compared at the same bulk densities, the CFthermal-acoustic insulation materials have greater tensile strengthsthan the materials made of glass fibers. In particular, a remarkabledifference between these two materials is noted in the tensile strengthsin the widthwise and thickness directions. More specifically, thetensile strength of the CF thermal-acoustic insulation material in thethickness direction is 8.5 times as that of the material made of glassfibers when the bulk density is 5 kg/m³, and is 15 times as that of thematerial made of glass fibers when the bulk density is 10 kg/m³.

Referring now to table 2, when each tensile strength ratio (percentage)of minimum to maximum, medium to maximum, and minimum to medium iscompared at the same bulk density, the CF thermal-acoustic insulationmaterials have larger values than the materials made of glass fibers(conventional material). That is, the difference between the tensilestrengths in each axis was smaller in the CF thermal-acoustic insulationmaterials. In particular, it is noted that the difference between thelengthwise direction and the widthwise direction was remarkably small inthe CF thermal-acoustic insulation materials. More specifically, thetensile strength ratio of the medium tensile strength direction to themaximum tensile strength direction was equal to or greater than 86% inthe CF thermal-acoustic insulation materials with a bulk density of 3kg/m³ to 10 kg/m³. The ratio of the minimum tensile strength directionto the maximum tensile strength direction was equal to or greater than5.4%.

In consideration of durability and handleability in manufacturing ormounting thermal-acoustic insulation materials, the difference oftensile strength between each direction in three axes should be renderedas small as possible. However, it is extremely difficult to eliminatethe difference of the tensile strengths due to certain factors inmanufacturing. The reason is that, when a method of producing awool-like carbon fiber aggregate by accumulating carbon fibers isemployed in manufacturing, many of the fibers composing the accumulatedmatter are oriented in the direction orthogonal to the gravitationaldirection because the fibers have a tendency to keep the orientationstate more stable against gravitation. In other words, the fibers tendto be oriented in the lengthwise or widthwise direction. Therefore, theresulting thermal-acoustic insulation material produced by bonding thefibers having the aforementioned orientation state, the tensilestrengths in the lengthwise and widthwise directions are likely to betoo large while the tensile strength in the thickness direction islikely to be too small.

If the tensile strength difference of a CF thermal-acoustic insulationmaterial is equal to or less than that of a conventional material,however, the handleability and durability thereof are guaranteed to beat least equal or superior to those of the conventional material. Inother words, judging from table 2, when the percentage of the minimum tothe maximum is controlled at 4% or higher, and the percentage of themedium to the maximum 35% or higher, the resulting material can achievethe handleability, mountability and durability equal or superior to theconventional material. From the results in table 2, it is understoodthat the CF thermal-acoustic insulation material with a bulk density of3 kg/m³ to 7 kg/m³ can meet these conditions.

The description below will now detail a manufacturing method of a CFthermal-acoustic insulation material having the physical properties asdescribed in the preceding sections.

First, anisotropic pitch obtained by polymerizing condensed polycyclichydrocarbon is prepared in accordance with a publicly known art(Japanese Unexamined Patent Publication No. 63-146920). Second, spunfibers are produced by melting the pitch, and then discharging it from aspinning nozzle and at the same time blowing a heated gas from aroundthe spinning nozzle in the same direction to which the melted pitch isdischarged (preferably to the direction parallel to the dischargedirection). The heated gas has a role to prevent the discharged pitchfrom being immediately cooled, and to obtain fibers having anappropriate length.

The aforementioned spun fibers are collected with, for example, a netand then subjected to infusibilizing treatment (oxidation treatment).Carbon fiber precursors are thereby produced. The carbon fiberprecursors are then subjected to carbonizing treatment in an inert gasat 650° C. or higher, and the resulting fibers are employed as carbonfibers.

In the above method, it is possible to control an average diameter andaverage fiber length of the spun fibers at a desired value by varying adischarge outlet diameter of the spinning nozzle within the range of 0.5mm to 0.2 mm and by adjusting the melt treatment temperature anddischarge speed of the pitch as well as the temperature and dischargespeed of the heated gas.

Although the diameters and lengths of the fibers slightly change afterinfusibilizing treatment and carbonizing treatment, there is practicallyno difference between the sizes of the spun fibers and those of theresulting carbon fibers, when the margin of error in the measurements istaken into account. Likewise, there is practically no difference intheir average fiber diameters and average fiber lengths.

With employing the aforementioned carbon fibers, a CF thermal-acousticinsulation material of the present invention is prepared in accordancewith the following manner.

First, a carbon fiber aggregate (sprayed and accumulated matter) whereina thermosetting resin solution is sprayed is prepared according toeither of the following methods. The first method is that the carbonfibers collected by a net and the like are opened by, for example,blowing the air, and then the opened fibers are dropped and accumulatedwith a thermosetting resin solution being sprayed to the fibers (sprayand accumulation method). The second method is that, after the carbonfibers are opened as in the above method, the opened fibers are droppedand accumulated onto a plane to form a coarse wool-like aggregate, andthereafter a thermosetting resin solution is sprayed to the aggregate(accumulation—spray method).

Second, slight pressure is applied to the above-mentioned sprayed andaccumulated matter with two pressing plates normally from the thicknessdirections, and then, with being kept in the pressed state, the matteris heated to cure the thermosetting resin A three-dimensional carbonfiber structure wherein the fiber contact points are bonded together bya thermosetting resin is thus formed. It is to be noted that thepressing plates may be applied from the directions orthogonal to thethickness directions.

It is to be understood that the CF thermal-acoustic insulation materialsof the present invention may comprise only the aforementioned carbonfibers and thermosetting resin, or may comprise the aforementionedcarbon fibers as a main constituent material and contain other fibers,insofar as the other fibers do not affect the acoustic and thermalinsulation properties of the resulting material. Some examples of suchfibers are; glass fibers, polyester fibers, ceramic fibers and the like.

More specifically, in the case where the amount of thermosetting resinto be added is represented by b kg/m³, for example, if a bulk density ofa carbon fiber aggregate is restricted at least within the range from(3-b) kg/m³ to less than (10-b) kg/m³, the resulting CF thermal-acousticinsulation material will have a bulk density within the range from 3kg/m³ to less than 10 kg/m³.

In this case, it is preferable to restrict a bulk density of a carbonfiber aggregate (the bulk density after omitting b) at less than 1.3kg/m³ by adjusting the gap between the aforementioned two pressingplates in the heating and forming step, in order to obtain a formedmatter (three-dimensional structure of carbon fibers) with a desiredbulk density. When the aggregate has a bulk density of less than 1.3kg/m³, which is quite coarse, the fibers therein are orientedsufficiently unevenly. Consequently, the resulting three-dimensionalcarbon fiber structure becomes bulky (i.e., has a small bulk density),and moreover, only the contact points of the fibers therein are bondedin the structure. The structure thus obtained has more unvaried tensilestrengths in the three-axis directions.

In preparing the aforementioned sprayed and accumulated matter, it ispreferable to drop the opened carbon fibers from the height of 100 cm toa plane. The fibers are thereby oriented unevenly without using specialequipment. The reason is, when the fibers are lightweight carbon fibershaving an average fiber diameter of 0.5 μm to 2 μm and an average fiberlength of 3 mm to 8 mm, and when the fibers are dropped from the heightof 100 cm, some fibers are oriented in the gravitational direction butsome in the orthogonal direction to it. Therefore, a bulky carbon fiberaccumulation (wool-like carbon fiber aggregate) with a random fiberorientation can be obtained. Accordingly, a three-dimensional carbonfiber structure with a random fiber orientation can be obtained byspraying thermosetting resin solution to this accumulated matter.

Regarding a method of dropping carbon fibers, the carbon fibers may bedropped by free-fall, or an air current may be applied upward (thedirection in which the falling speed is decreased) or downward (thedirection in which the falling speed is increased). If the air currentis applied, it is easier to obtain a fiber aggregate having a desiredbulk density since the fiber orientation can be controlled.

In the case where the fibers are gradually dropped onto a plane byfree-fall, as in snowfall, the single fibers are oriented quite unevenlyin each direction in three axis. Even so, however, many fibers tend toorient in the direction parallel to the gravitation (the lengthwise orwidthwise direction), resulting the tensile strength in the thicknessdirection smaller than the tensile strengths in the lengthwise orwidthwise direction. If a particularly larger tensile strength in thethickness direction is required, a method in which pressure is appliedfrom the lengthwise and/or widthwise direction with a press machine maybe employed in the pressing step. When pressure is applied from thesedirections, the material with little difference between the tensilestrengths in the three-axis direction can be obtained.

Among the thermosetting resins which may be employed in theaforementioned step are, for example, phenolic resins, melamine resinsand silicone resins. The amount to be used is normally 10 wt. % to 40wt. % to the CF thermal-acoustic insulation material, or more preferably20 wt. % to 30 wt. %. It is not preferable if the value exceeds 40 wt. %because the portions of the fibers except the contact points will bebonded due to the excessive amount of the binder. On the other hand, ifthe value is less than 10 wt. %, the contact points will beinsufficiently bonded and thereby resulting too small tensile strengthsand compression recovery rates.

Regarding to phenolic resins, a heat treatment temperature in theaforementioned heating and forming step should be 150° C. to 250° C.,normally 180° C. to 220° C.

In accordance with the manufacturing method described thus far, the CFthermal-acoustic insulation materials of the present invention havingthe properties as described in the preceding sections can be obtained.

Based on the examples, the further detailed description of the presentinvention will be given hereinafter.

EXAMPLES 1-4

A pitch with a softening point of 280° C. was obtained by polymerizingcondensed polycyclic hydrocarbon, and the pitch was melted at 320° C.The melted pitch was discharged out of a spinning nozzle having adischarge hole with a diameter of 0.25 mm, and at the same time a heatedgas with a temperature of 320° C. was blown in the direction the same asand parallel to the pitch discharging direction. The pitch was spun andthereby formed into fibers, and then collected with a net. The carbonfiber precursors thus obtained had a diameter from about 0.5 to 3.5 μm(an average fiber diameter of 1.3 μm) and a fiber length of 1 to 15 mm(an average fiber diameter of 5 mm). FIG. 1 shows a diameterdistribution of the carbon fibers produced according to the aboveconditions.

Subsequently, the resulting fibers were infusibilized by heating in theair for 30 minutes at 300° C., and then carbonized by heating in aninert gas for 30 minutes at a predetermined temperature (650° C., 700°C., 750° C. or 800° C.). Four types of carbon fibers each carbonized ata different temperature were thus obtained. The diameters and fiberlengths of these carbon fibers were approximately equal to theaforementioned carbon fiber precursors.

Four types of CF thermal-acoustic insulation materials were preparedfrom the aforementioned four types of carbon fibers. More specifically,the carbon fibers were opened by blowing the air, and then the openedfibers were little by little dropped from a height of 100 cm onto aplane by free-fall, like the way snow falls, and a wool-like aggregate(non-bonded state) with a thickness of 120 mm and a bulk density of 0.7kg/m³ was thus obtained.

Subsequently, 20 wt. % phenolic resin solution whose amount was 150 wt.% based on the weight of this carbon fiber aggregate was sprayed to theaggregate, and then the aggregate was pressed with a pressing machineequipped with two pressing plates until the thickness of the aggregatebecame about 25 mm. (No pressure was applied to the lengthwise andwidthwise directions.) The aggregate was heated to 200° C. with beingheld in the pressed state, and the phenolic resin was thus completelycured. Accordingly, three-dimensional carbon fiber structures (4different types) having a bulk density of 4.8 kg/m³ and a size of 1.5 mlong, 0.5 m wide and 25 mm thick were produced, and the resultingstructures were employed as CF thermal-acoustic insulation materials.

EXAMPLE 5

A CF thermal-acoustic insulation material of Example 5 with a bulkdensity of 4.0 kg/m³ was prepared in the same manner as Examples 1through 4 except that the carbonizing temperature was 700° C. and thethickness of the carbon fiber aggregate was 100 mm. The size andthickness of the material thus produced were the same as those of thematerials produced according to Examples 1 through 4.

EXAMPLE 6

The CF thermal-acoustic insulation material of Example 6 having a bulkdensity of 7.0 kg/m³ was prepared in the same manner as Examples 1through 4 except that the carbonizing temperature was 700° C. and thethickness of the carbon fiber aggregate was 175 mm. The size andthickness of the material thus produced were the same as those of thematerials produced according to Examples 1 through 4.

COMPARATIVE EXAMPLE 1

The CF thermal-acoustic insulation material of Comparative Example 1 wasprepared in the same manner as Examples 1 through 4 except that thecarbonizing temperature was 850° C. The size, thickness ard bulk densityof the material thus produced were the same as those of the materialsproduced according to Examples 1 through 4.

COMPARATIVE EXAMPLE 2

The CF thermal-acoustic insulation material of Comparative Example 2 wasprepared in the same manner as Examples 1 through 4 except that thecarbonizing temperature was 900 (C. The size, thickness and bulk densityof the material thus produced were the same as those of the materialproduced according to Examples 1 through 4.

COMPARATIVE EXAMPLE 3

The CF thermal-acoustic insulation material of Comparative Example 3 wasprepared in the same manner as Examples 1 through 4 except that carbonfibers employed for this example were produced from isotropic pitch madefrom coal tar and carbonized at 950° C., and the carbon fibers had afiber diameter of 13 μm and an average fiber diameter of 25 mm.

COMPARATIVE EXAMPLE 4

The glass fiber thermal-acoustic insulation material of ComparativeExample 4 (thickness: 25 mm, bulk density: 6.7 kg/m³) was prepared fromglass fibers having an average fiber diameter of 1.0 μm and an averagefiber length of 10 mm, and by bonding the fibers with phenolic resin.

Each type of the thermal-acoustic insulation material of Examples 1through 6 and Comparative Examples 1 through 4 was tested according tothe same methods as described in “THE BEST MODE FOR CARRYING OUT THEPRESENT INVENTION” hereinbefore. Accompanied with the manufacturingconditions of the carbon fibers, the test results are set forth inTables 3 and 4.

TABLE 3 Properties of three- dimensional structures Conditions of fibers(thermal-acoustic Carbon- insulation materials) izing Gal- AverageAverage temper- Bulk vanic diameter length ature density currentGalvanic μm mm ° C. kg/m³ μA corrosion Example 1 1.3 5 800 4.8 17 ±Example 2 1.3 5 750 4.8 1.8 − Example 3 1.3 5 700 4.8 0.1 − Example 41.3 5 650 4.8 0 − Example 5 1.3 5 700 4.0 0.1 − Example 6 1.3 5 700 7.00.1 − Com- 1.3 5 850 4.8 56 + parative Example 1 Com- 1.3 5 900 4.8110 + parative Example 2 Com- 13 25 950 12.0 36 + parative Example 3Com- 1.0 10 N/A 6.7 0 − parative Example 4

TABLE 4 Acoustic Com- Tensile absorptivity pression strength % re-Tensile ratio Thermal (25 mm) covery strength (%) conductivity 500 1000rate g/mm² P2/P1 P3/P1 W/m · ° C. Hz Hz % Example 1 0.6 100  5.1 0.03720 52 82 Example 2 1.1 82 5.3 0.036 22 55 86 Example 3 1.6 88 6.3 0.03520 52 91 Example 4 1.1 100  5.5 0.036 21 53 85 Example 5 1.2 83 5.00.037 18 50 83 Example 6 2.2 95 5.1 0.033 27 60 93 Com- 1.1 * * 0.037 2152 84 parative Example 1 Com- 0.8 * * 0.037 21 53 83 parative Example 2Com- 3.6 * * 0.060 11 22 78 parative Example 3 Com- 0.9 33 2.5 0.039 1648 70 parative Example 4 P1 Tensile strength in the lengthwise direction(Maximum tensile strength) P2 Tensile strength in the widthwisedirection P3 Tensile strength in the thickness direction (Minimumtensile strength) *Not measured

As apparent in table 3, the galvanic currents in Examples 1 through 5are 0.2 μA to 17 μA, and no galvanic corrosion was observed in Examples2 through 6 although a very little galvanic corrosion was recognized inExample 1.

On the contrary, galvanic corrosion was obviously recognized inComparative Example 1 (carbonizing treatment: 850° C., galvanic current:56 μA), Comparative Example 2 (carbonizing treatment: 900° C., galvaniccurrent: 110 μA), and Comparative Example 3 (carbonizing treatment: 950°C., galvanic current: 36 μA). The reason why no galvanic corrosion wasrecognized in Comparative Example 4 is that glass fibers do not generategalvanic current.

As shown in table 4, the tensile strength ratios (percentages) ofExamples 1 through 6 were P2/P1=82−100, and P3/P1=5.6−6.3. Compared tothese, in Comparative Example 4, the ratios are P2/P1=33 and P3/P1=2.5,which indicates the material of Comparative Example 4 has far greaterdifference in the tensile strengths in three axes than that of the CFthermal-acoustic insulation material of the present invention.

As to thermal conductivity (W/m·° C.), 0.035 to 0.037 were the thermalconductivities in Examples 1 through 4, in which the materials having abulk density of 4.8 kg/m³ were employed. 0.037 was the value in Example5, in which the material having a bulk density of 4.0 kg/m³ wasemployed, and 0.033 was the value in Example 6, in which the materialhaving a bulk density of 7.0 kg/m³ was employed. Compared to these,0.039 was the value in Comparative Example 4 (the thermal-acousticinsulation material made of glass fibers), in which the material havinga bulk density of 6.7 kg/m³ was employed. These results demonstrate thatthe CF thermal-acoustic insulation materials of the present inventioncan achieve higher thermal insulation quality with a smaller bulkdensity when compared to the material made of glass fibers. It is to benoted that the smaller the thermal conductivity is, the better thethermal insulation quality.

As to vertical incident acoustic absorptivities at 1000 Hz (%) in athickness of 25 mm, 52 to 55 are the values in Examples 1 through 4 withthe materials having a bulk density of 4.8 kg/m³, 50 in Example 5 withthe material having a bulk density of 4.0 kg/m³, 60 in Example 6 withthe material having a bulk density of 7.0 kg/m³. Compared to these, 48is the value in Comparative Example 4 (the thermal-acoustic insulationmaterial made of glass fibers) with the material having a bulk densityof 6.7 kg/m³. As similar to the above results, these results alsodemonstrate that the CF thermal-acoustic insulation materials of thepresent invention can achieve greater sound insulating effect with asmaller bulk density when compared to the material made of glass fibers.

INDUSTRIAL APPLICABILITY

As has been described, the present invention can attain each of thepreviously-mentioned objects satisfactorily. A thermal-acousticinsulation material having high degree of thermal and acousticinsulation quality as well as excellent tensile strength and compressionresilience can be obtained in accordance with the present invention.Furthermore, since the thermal-acoustic insulation material of thepresent invention comprises carbon fibers as a main constituentmaterial, the resulting material has favorable properties intrinsic tocarbon fibers such as lightness, chemical stability, incombustiblity,non-hygroscopicity and such properties that the material do not generatetoxic fumes in case of fire. Moreover, the material of the presentinvention retains improved qualities in galvanic corrosiveness andnon-electrical conductivity, both of which are drawbacks of thethermal-acoustic insulation material comprising carbon fibers. Inaddition, the material of the present invention possesses remarkablyimproved mechanical characteristics such as excellent tensile strengthand compression recovery rate.

The thermal-acoustic insulation material as such not only excels inthermal and acoustic insulation quality in its initial mounting, butalso does not suffer the deterioration of the quality even after longuse. Further, the material does not cause galvanic corrosion to themembers surrounding it and does not bring about short circuits resultingfrom the material itself or the fibers detached from the material in theelectric circuits thereabout.

The present invention has a significant value in industrialapplicability in that it can provide a CF thermal-acoustic insulationmaterial not only usable for a member capable of saving energyconsumption in housing and the like, but also suitably usable foraircraft, high-speed train cars, spacecraft and the like, whereinconstant vibration exists, a great deal of metal material is used, andvarious electrical equipment is mounted.

1. A thermal acoustic insulation material comprising: a multiplicity ofanisotropic pitch-based carbon fibers having an average fiber diameterof 1.3 μm or less and an average fiber length of 1 mm to 15 mm, saidcarbon fibers being non-galvanic corrosive and being bonded by athermosetting resin at contact points of said carbon fibers so as toform a carbon fiber aggregate having a bulk density of from 3 kg/m³ to10 kg/m³; wherein said thermal-acoustic insulation material isnon-galvanic corrosive.
 2. A thermal-acoustic insulation material as inclaim 1, wherein said thermal-acoustic insulation material shows agalvanic current of 10 μA or lower in a galvanic cell having anelectrode made of said thermal-acoustic insulation material, anotherelectrode made of an aluminum plate, and an electrolytic solution of0.45 wt. % aqueous sodium chloride solution.
 3. A thermal-acousticinsulation material as in claim 2, which has a maximum tensile strengthof 1.0 g/mm² or higher.
 4. A thermal-acoustic insulation material as inclaim 2, which has a compression recovery rate of 70% or higher.
 5. Athermal-acoustic insulation material as in claim 2, wherein a minimumtensile strength of the orthogonal direction to said maximum tensilestrength is 0.04 times or higher as said maximum tensile strength and,at the same time, a tensile strength of the orthogonal direction to boththe direction of said maximum tensile strength and the direction of saidminimum tensile strength is 0.76 times or higher as said maximum tensilestrength.
 6. A thermal-acoustic insulation material as in claim 2, whichhas a thermal conductivity of 0.039 W/m·° C. or lower.
 7. Athermal-acoustic insulation material as in claim 2, wherein a verticalincident acoustic absorptivity at a frequency of 1000 Hz of saidthermal-acoustic insulation material with a thickness of 25 mm is 48% orhigher.
 8. A thermal-acoustic insulation material as in claim 2, whereinsaid carbon fibers are produced from anisotropic pitch obtained bypolymerizing condensed polycyclic hydrocarbon.
 9. A thermal-acousticinsulation material as in claim 1, wherein said anisotropicpitched-based carbon fibers have an average fiber diameter of from 0.5μm to 1.0 μm.
 10. A thermal-acoustic insulation material as in claim 9,which has a maximum tensile strength of 1.0 g/mm² or higher.
 11. Athermal-acoustic insulation material as in claim 9, which has acompression recovery rate of 70% or higher.
 12. A thermal-acousticinsulation material as in claim 9, wherein a minimum tensile strength ofthe orthogonal direction to said maximum tensile strength is 0.04 timesor higher as said maximum tensile strength and, at the same time, atensile strength of the orthogonal direction to both the direction ofsaid maximum tensile strength and the direction of said minimum tensilestrength is 0.76 times or higher as said maximum tensile strength.
 13. Athermal-acoustic insulation material as in claim 9, which has a thermalconductivity of 0.039 W/m·° C. or lower.
 14. A thermal-acousticinsulation material as in claim 9, wherein a vertical incident acousticabsorptivity at a frequency of 1000 Hz of said thermal-acousticinsulation material with a thickness of 25 mm is 48% or higher.
 15. Athermal-acoustic insulation material as in claim 9, wherein said carbonfibers are produced from anisotropic pitch obtained by polymerizingcondensed polycyclic hydrocarbon.
 16. A thermal-acoustic insulationmaterial as in claim 1, which has a maximum tensile strength of 1.0g/mm² or higher.
 17. A thermal-acoustic insulation material as in claim16, which has a compression recovery rate of 70% of higher.
 18. Athermal-acoustic insulation material as in claim 16, wherein a minimumtensile strength of the orthogonal direction to said maximum tensilestrength is 0.04 times or higher as said maximum tensile strength and,at the same time, a tensile strength of the orthogonal direction to boththe direction of said maximum tensile strength and the direction ofminimum tensile strength is 0.76 times or higher as said maximum tensilestrength.
 19. A thermal-acoustic insulation material as in claim 16,which has a thermal conductivity of 0.039 W/m·° C. or lower.
 20. Athermal-acoustic insulation material as in claim 16, wherein a verticalincident acoustic absorptivity at a frequency of 1000 Hz of saidthermal-acoustic insulation material with a thickness of 25 mm is 48% orhigher.
 21. A thermal-acoustic insulation material as in claim 16,wherein said carbon fibers are produced from anisotropic pitch obtainedby polymerizing condensed polycyclic hydrocarbon.
 22. A thermal-acousticinsulation material as in claim 1, which has a compression recovery rateof 70% or higher.
 23. A thermal-acoustic insulation material as in claim22, wherein a minimum tensile strength of the orthogonal direction tosaid maximum tensile strength is 0.04 times or higher as said maximumtensile strength and, at the same time, a tensile strength of theorthogonal direction to both the direction of said maximum tensilestrength and the direction of minimum tensile strength is 0.76 times orhigher as said maximum tensile strength.
 24. A thermal-acousticinsulation material as in claim 22, which has a thermal conductivity of0.039 W/m·° C. or lower.
 25. A thermal-acoustic insulation material asin claim 22, wherein a vertical incident acoustic absorptivity at afrequency of 1000 Hz of said thermal-acoustic insulation material with athickness of 25 mm is 48% or higher.
 26. A thermal-acoustic insulationmaterial as in claim 22, wherein said carbon fibers are produced fromanisotropic pitch obtained by polymerizing condensed polycyclichydrocarbon.
 27. A thermal-acoustic insulation material as in claim 1,wherein a minimum tensile strength of the orthogonal direction to saidmaximum tensile strength is 0.04 times or higher as said maximum tensilestrength and, at the same time, a tensile strength of the orthogonaldirection to both the direction of said maximum tensile strength and thedirection of said minimum tensile strength is 0.76 times or higher assaid maximum tensile strength.
 28. A thermal-acoustic insulationmaterial as in claim 27, which has a thermal conductivity of 0.039 W/m·°C. or lower.
 29. A thermal-acoustic insulation material as in claim 27,wherein a vertical incident acoustic absorptivity at a frequency of 1000Hz of said thermal-acoustic insulation material with a thickness of 25mm is 48% or higher.
 30. A thermal-acoustic insulation material as inclaim 27, wherein said carbon fibers are produced from anisotropic pitchobtained by polymerizing condensed polycyclic hydrocarbon.
 31. Athermal-acoustic insulation material as in claim 1, which has a thermalconductivity of 0.039 W/m·° C. or lower.
 32. A thermal-acousticinsulation material as in claim 31, wherein a vertical incident acousticabsorptivity at a frequency of 1000 Hz of said thermal-acousticinsulation material with a thickness of 25 mm is 48% or higher.
 33. Athermal-acoustic insulation material as in claim 31, wherein said carbonfibers are produced from anisotropic pitch obtained by polymerizingcondensed polycyclic hydrocarbon.
 34. A thermal-acoustic insulationmaterial as in claim 1, wherein a vertical incident acousticabsorptivity at a frequency of 1000 Hz of said thermal-acousticinsulation material with a thickness of 25 mm is 48% or higher.
 35. Athermal-acoustic insulation material as in claim 34, wherein said carbonfibers are produced from anisotropic pitch obtained by polymerizingcondensed polycyclic hydrocarbon.
 36. A thermal-acoustic insulationmaterial as in claim 1, wherein said carbon fibers are produced fromanisotropic pitch obtained by polymerizing condensed polycyclichydrocarbon.
 37. A method of manufacturing thermal-acoustic insulationmaterial, comprising the steps of: producing spun fibers having anaverage fiber diameter less than 2 am and an average fiber length of 1mm to 15 mm by heating and melting anisotropic pitch obtained bypolymerizing condensed polycyclic hydrocarbon, then discharging a meltedmatter out of a spinning nozzle and at the same time, blowing a heatedgas from around the spinning nozzle in the same direction in which themelted matter is discharged; manufacturing non-galvanic corrosive carbonfibers by infusibilizing said spun fibers and thereafter carbonizingsaid spun fibers at not lower than 550° C. but lower than 800° C.;forming a carbon fiber aggregate having a bulk density less than 1.3kg/m³ by aggregating said non-galvanic corrosive carbon fibers; sprayinga thermosetting resin solution to the carbon fiber aggregate; and curingthe thermosetting resin by compressing and heating the carbon fiberaggregate sprayed with the thermosetting resin solution to bond contactpoints of said carbon fibers and thereby manufacture a three dimensionalstructure of carbon fibers having a bulk density of from 3 kg/m³ to 10kg/m³.
 38. A method of manufacturing a thermal-acoustic insulationmaterial as in claim 37, wherein in said step of forming a carbon fiberaggregate, said non-galvanic corrosive carbon fibers are opened by theair and dropped from a height of at least 100 cm or higher onto a plane.39. A method of manufacturing a thermal-acoustic insulation material asin claim 37, wherein a temperature of carbonizing the spun fibers is notlower than 650° C. but lower than 750° C.
 40. A method of manufacturinga thermal-acoustic insulation material, comprising the steps of:producing spun fibers having an average fiber diameter of 1.3 μm or lessand an average fiber length of 1 mm to 15 mm, said producing comprisingheating and melting anisotropic pitch obtained by polymerizing condensedpolycyclic hydrocarbon to produce melted pitch, and discharging saidmelted pitch out of a spinning nozzle while at the same time, blowing aheated gas from around the spinning nozzle in the same direction thatthe melted pitch is discharged, to form said spun fibers; manufacturingnon-galvanic corrosive carbon fibers comprising infusibilizing said spunfibers to form infusibilized fibers and carbonizing said infusibilizedfibers at not lower than 550° C. but lower than 800° C., to form saidnon-galvanic corrosive carbon fibers; forming a carbon fiber aggregatecomprising aggregating and compressing said non-galvanic corrosivecarbon fibers to a bulk density of from (3−b) kg/m³ to (10−b) kg/m³, toform said carbon fiber aggregate; spraying a thermosetting resinsolution to said carbon fiber aggregate so that the amount of athermosetting resin in relation to the amount of the carbon fiberaggregate is b, where b is an arbitrary number fixed so that the bulkdensity is positive and the relationship 0.3≦b≦4 is satisfied, to form asprayed aggregate; and curing said thermosetting resin comprisingheating said sprayed aggregate, to form a three-dimensional structure ofcarbon fibers, wherein said carbon fibers are bonded at contact pointsthereof, and said three-dimensional structure has a bulk density of from3 kg/m³ to 10 kg/m³.
 41. The method of manufacturing a thermal-acousticinsulation material as in claim 40, wherein said carbonizing is carriedout at a temperature of not lower than 650° C., but lower than 750° C.